A grouping of coral islands is known as an atoll. Atolls can be a collection of small islands or one continuous island. They form from the remnants of undersea volcanoes.
A group of coral islands, or an atoll, form as underwater volcanoes grow and get closer to the surface, effectively raising the sea floor. Once within reach of sunlight in sufficient amounts, corals and other marine life take hold. As volcanoes erode due to wave action and wind, the hard corals remain. Because of this, most atolls are ring-shaped. Islands form over time as corals die and their hard exteriors break down into the sand. Coral islands have distinguishing characteristics, such as high elevations, reef-front terraces and mostly submerged reefs.
The ring shape of the atoll is not always at a consistent height, as different areas of the volcano erode at different times. In such cases, a collection of coral islands is formed. Additionally, if a large section of the volcano falls away, crescent-shaped atolls begin to develop. Because of the way they are formed, atolls partially or completely enclose a central lagoon. These lagoons are oases for marine life, as they are sheltered from the harsh conditions of the open ocean.